Old Earth Ministries Online Geology Curriculum

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Geology - Chapter 9: Plate Tectonics

Although the concept of
the continental land masses moving originated in the 1800's, it was not
until the 1960's that the theory of plate tectonics was developed. In
the 1960's, the ocean floors were mapped, and core drilling through the
ocean floors examined the magnetic and seismic characteristics of the floor.
Now, with exact laser and global positioning system (GPS) measurements,
plate tectonics has moved from a theory to a proven fact backed by
scientific measurements.

This lesson plan is designed
so that your child can complete the chapter in five days. The only
decisions you will need to make will be concerning the research task for
Tuesday. It is up to you to determine if the student will simply fill
in the answers, or provide a short essay answer. You will also need to
determine the percentage that this research will play in the overall chapter
grade, if any.

Why did God create a world
that involved moving plates? As you will find out, tectonic plates are
responsible for building mountains, and they are responsible for volcanic
activity. These two methods of building new land counteracts the
erosion that occurs, which removes land. Without a method of replacing
land lost to erosion, our world would be flat and completely covered in
water...great if you are a fish, but not so great for us humans! The
system God put in place ensures that the land will renew itself, and give us
dry land to live on!

Continental Drift

When maps were first
made of the continental land masses, scientists noted that the continents
looked like they could fit together, like pieces of a puzzle. Although
this occurred in the 1800's, the first proposal that gained attention was
put forth by Alfred Wegener, who was a German meteorologist. In 1915
he
authored a work called The Origin of Continents and Oceans. In a
series of maps, he showed how the land masses had moved, and had originated
from a single landmass, or supercontinent. He called this landmass Pangea
(also spelled Pangaea). The name stuck, and geologists today still
call the last single landmass by that name. Interestingly, at the time
it was proposed, most geologists rejected the idea until the 1960's.

There is several
good lines of evidence supporting Wegener's original theory.

Paleontologic Evidence.
Wegener and others noted that certain fossils found on both sides of the
Atlantic ocean were very similar. From this evidence, they reasoned
that Africa and South America had once been connected. Marine fossils
could have migrated across the ocean, but plants and land-dwelling reptiles
could not. Consider the seed fern Glossopteris, which
has been found in rocks of the same age in all of the southern continents.
Fossils have been found in South America, South Africa, Australia, and
India, and even in Antarctica. The seeds of this plant are too large
to be carried by the wind. For reptiles, the distribution of Paleozoic
and Mesozoic species is even more compelling. For example, the
mammal-like reptile Lystrosaurus was a land-dwelling reptile.
Its fossils are abundant in South Africa, South America, Asia, and
Antarctica. For a diagram of how other fossils support the concept of
Pangea, see the diagram at right.

Geologic Features and Rock Types.
Some geologic features on continents match, even though they are thousands
of miles apart. For example, folded mountain ranges at the southern
tip of Africa terminate sharply at the coast, and an equivalent structure of
the same age, with the same style of deformation, and the same rock type,
appears in Argentina. This is also true of the Appalachian Mountains.
They extend northeastward into Newfoundland and terminate abruptly on the
coast. They continue on the coast of Ireland and Brittany.

Glaciation. Near the end
of the Paleozoic Era, glaciers covered a large area of the southern
hemisphere. The deposits left by these glaciers are easily identified,
and they are present on parts of South America, Africa, Australia, and
Antarctica. Striations (groves left in the rocks from the scraping
action of ice) give the direction in which the ice moved, which further
confirms the way the continents fit together. For more, see
Karoo
Ice Age.

Paleoclimates. The climate of the past
can be used as evidence of tectonic movement. For example, there are
vast coal deposits in Antarctica, which is now a frozen continent.
This indicates it once had abundant plant life and a more hospitable
climate. Other continents which have ancient deposits of desert
sandstones are hard to explain based on their present positions, yet when
combined with where the continents were located millions of years ago, they
are easily explained.

Why did the
geologists of the early 1900's reject the theory of continental drift, when
the evidence seemed to support it? They reasoned that there was no
mechanism that could cause the such large continental land masses to move.
This changed with the study of the earth's ocean basins in the 1960's.

Plate Tectonics is Born

With the study of
the ocean floor in the 1960's, geologists noticed features that clearly
favored the theory of continental drift. These features were the
topography and geology of the ocean floor, and rock magnetism.

Ocean Floor Geology

New devices
developed in the 1950's and 1960's enabled geologists to map the topography
of the ocean floor. The topography revealed that the ocean basins are
divided by long ridges. These features were studied in an earlier
chapter. Another
tool used by geologists is called core
drilling. Using a cylindrical drill bit that is hollow,
geologists are able to retrieve sections of the ocean floor to see what it
is composed of, and to make other measurements, such as radiometric dating.
It was noticed that the younger rocks are near the ridges, and the further
you got away from the ocean ridges, the older the rocks. Geologists
finally concluded that the ocean floors were spreading from the mid-ocean
ridges, and they dived under the continental land masses at the deep
trenches that were observed in the topographic maps of the ocean floor.
They reasoned that because the mantle was partially molten, convection
currents in the mantle had to be the mechanism that caused this movement.
Based on the rate of movement, obtained from the dating of the oceanic
floor, it was discovered that the oceanic floor completely regenerated
itself in 200 to 300 million years.

Also in the 1950's, advances in magnetometers, the tools used to measure
magnetism in rocks, brought further evidence of plate movement. The
rock type being produced at the mid-oceanic ridges is basalt, which contains
iron. As the rock cools, the mineral grains become oriented based on
the magnetic field. Because of this, a new branch of study arose,
called paleomagnetism, which is the study of magnetism of the
past. However, it was not just igneous basalt that could be used.
Red sandstone also contains iron, so that its grains become oriented also.
By studying the orientation of these grains in basalt and sandstone,
geologists are able to put together a history of the earth's magnetic field.

It was discovered that over the millions of years of earth's history, that
the earth's magnetic poles actually wander (called polar wandering).
You may have already noticed evidence of this. If you look at a map,
you may notice that there is a geographic north and south pole, from which
the grids of latitude and longitude are determined. The
geographic poles are determined from the point of rotation of the earth.
The axis upon which the earth rotates indicates the geographic poles.
The actual magnetic poles are different from the geographic poles.
However, it is even more complicated than that! For more, see
North Pole. To see a map of the history of the North Magnetic Pole
for the last 400 years, see the image at right.

Another feature that geologists noted is that the grains were oriented in
the opposite direction in many rocks. Some would indicate an
orientation to the north ("positive" for the magnetic field), whereas others
a short distance away were oriented toward the south. Geologists
determined that the earth's magnetic field reversed itself from time to
time. These magnetic reversals occurred frequently.
For example, during the last 76 million years, there have been at least 171
reversals. Our current polarization has been in effect for the last
700,000 years. The periods of alternating polarity are called
polarity epochs, and they average about one million years in
duration. The changing polarity can be mapped on the ocean floor,
which produces a striped pattern of alternating periods of normal and
reversed polarity (see picture at right). The striping pattern on one
side of the oceanic ridge is a mirror image of the pattern on the other side
of the ridge. To read more, click
this link.

More evidence for plate tectonics comes from the sediments on the ocean
floor. Thanks to core drilling, scientists can also examine the
fossils that are buried in a particular section of ocean floor.
Fossils with known ranges of existence confirm the dates that we obtain from
radiometric dating. For example, if we radiometrically date a section
of ocean floor basalt at 100 million years old, then the fossils that are
contained just above the basalt should be close to the same age. When
we examine them, we find that this is the case.

In addition, observed rates of sedimentation in the open ocean are about 1.0
centimeters of sediment every 1,000 years. If the ocean basins had
existed 500 million years ago, the sediment would be at least 5 kilometers
thick. However, ocean bed sediment is not this thick. In fact,
the thickest sediments are only 300 meters thick. The oldest ocean
sediment dates to about 160 million years old. By comparison, the
oldest continental crust material is dated to about 4 billion years old.

Plate Geography

The earth's crust is made up of tectonic plates that fit together like the
pieces of
a puzzle. The largest plate is the Pacific plate, which is composed
entirely of oceanic crust. It covers about one-fifth of the earth's
surface. The rest of the major plates are a combination of both
oceanic and continental crust. It is important to note that tectonic
plates are not permanent features of the earth's surface. As the
earth's surface changes, plates appear and disappear.

Plate Boundaries

Tectonic activity is mostly found at the plate boundaries, where two plates
intersect each other. There are three kinds of plate boundaries, each
having its own characteristics and causing specific rock deformation.

Divergent Plate Boundary

Also known as spreading centers, these are where a plate splits and is
pulled apart. Divergent plate boundaries are
characterized by tensional stresses, producing block faulting, fractures,
and open fissures at the spreading center. The rock type is typically
basaltic magma. For two animations showing this process, see Divergent
plate boundary
and
Divergent boundary.

Divergent plate boundaries can also occur on continents.
The Great Rift Valley
in Africa is an example (see map). Long, linear valleys, some
filled with water, and volcanoes are evidence of continental rifting.
The great volcanoes of Mount Kenya and
Mount Kilimanjaro owe their origin to continental rifting.

Convergent Plate Boundary

Convergent plate boundaries consist
of three types of plate collisions, based on the type of crust colliding and
its density. First, when the two colliding plates are oceanic, one
plate is thrust under the other. The lower plate dives, or "subducts"
underneath the other plate, hence the term subduction zones.
Second, subduction zones can also occur between continental and oceanic
plates. The oceanic plate always subducts under the continental plate,
because continental rock is less dense than oceanic rock, causing it to
float over the more dense oceanic crust.
Subduction zones are known for their volcanic activity.
Mount
Saint Helens in Oregon owes its origins to a subduction zone (see map at
right). For an animation of oceanic/continental convergence, see
Convergent plate boundary. Finally, in a collision of two continental crusts, neither crust can
subduct. In this case, one crust may briefly override the other.
In most cases, this collision produces mountain ranges, such as the
Himalayan range. The India land mass gradually moved northeast until
it collided with the Eurasian Plate. It is currently moving at a rate
of 67 mm/year, or about 2.6 inches.

Transform Fault Boundary

The third type of plate boundary is the transform fault boundary.
Some geologic texts also refer to them as passive plate margins. These
are zones where two plates slide past each other without diverging or
converging. Another term to describe their relationship is a
strike-slip fault. Unlike the other two types of boundaries, there is
no volcanic activity with transform faults. You will learn more about
these faults in the chapter on earthquakes.

Supercontinent History

During earth's history several supercontinents have been identified.
The youngest of these is Pangea, which existed 200 million years ago.
Evidence suggests that another supercontinent, named
Pannotia,
may have existed about 600 million years ago. Pannotia existed for
about 50 million years. The supercontinent
Rodinia
is believed to have formed about 1.1 billion years ago, and it existed until
about 750 million years ago. Some scientists have proposed another
supercontinent, called
Columbia, between 1.8 and 1.5 billion years ago. There may have
been earlier supercontinents as well.

In addition there have been other smaller supercontinents.
The most well-known of these is known as
Gondwana,
which comprised the landmasses of the continents that are now in the
southern hemisphere.

Research the answers to the following questions about plate tectonics.
Your parents may have you simply answer the questions, or they may have you
put it in essay form. Please follow your parents instructions.

To answer these questions, utilize a search engine to locate the best
webpages, or consult a textbook/encyclopedia. You may also use the
links at the bottom of this page.

Today you will
complete a 10 question practice quiz. The link to the quiz will open a
new window. You can come back here and check your answers. Do
not click the Back button on your browser during the quiz. After the
quiz, continue your research project, if necessary.